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Date: Tue, 17 Dec 2013 14:56:59 +0100
From: Gabriel Paubert <paubert AT iram DOT es>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] pcb: solder mask clearance in DRC?
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On Tue, Dec 17, 2013 at 01:41:53PM +0100, Stefan Salewski wrote:
> On Tue, 2013-12-17 at 10:02 +0000, Matt Rhys-Roberts wrote:
> > Hi all,
> > 
> > Is it possible to somehow cause the DRC to flag up when solder mask 
> > clearance is too small for manufacture?
> > 
> > I have inherited several SO8 footprints to choose from, and I didn't 
> > notice that the one I picked had barely any SM clearance around the 
> > pads. This cost us an extra 5% when the board makers detected it was 
> > outside the standard specs for their process.
> > 
> > No blame, just nice to see this added to the next generation DRC.
> > 
> > Any comments please?
> 
> Indeed in my opinion solder mask clearance is more a board property than
> a property of individual components on that board.

I have to disagree, for the finest pitch BGA, the solder mask aperture
is actually smaller than the pad. The pads are said to be SMD 
(solder mask defined), while traditional pads (defined by the copper
area) are said to be NSMD (non solder mask defined).

This said a few simple checks can be performed with single line commands,
typically using awk. For example, in my latest board, all components are
metric, so I can have the list of all the SMD cleareances I've used
by typing:

awk -e '/Pad/{print $7-$5}' boardname.pcb|sort -u

0.15
0.16
0.2

the smallest number is 0.15 (mm), which is what my board manufacturer asks.

If you have a mixture of mil and mm, it will be more complex but it's
rather easy to perform this kind of checks thanks to the simple, line
based, PCB file format.

Note that on this board 'grep "Pad.*mil" boardname.pcb' does not return 
anything, so I'm sure that all the pads are metric.

	Regards,
	Gabriel